1. Field of the Invention
One aspect of the present invention relates to a telephoto zoom lens suited for a single-lens reflex camera and, more particularly, to a compact telephoto zoom lens which has a focal length at the wide-angle end, which is about 1.4 to 2 times of the diagonal length of an effective frame, and has a zoom ratio exceeding .times.2.5. Another aspect of the present invention relates to a compact inner-focus type zoom lens which is suited for an auto-focus type single-lens reflex camera and has a zoom ratio exceeding .times.2.5.
2. Related Background Art
As conventional telephoto zoom lenses having a zoom ratio exceeding .times.2.5, (1) a four-group afocal zoom lens comprising positive, negative, positive, and positive lens groups, (2) a double telephoto type four-group zoom lens comprising positive, negative, positive, and negative lens groups, (3) a three-group zoom lens comprising positive, negative, and positive lens groups, and the like have been proposed. The four-group afocal zoom lens is suitable for a high-performance lens since the functions of the four lens groups are clearly distinguished from each other. The double telephoto type zoom lens is suitable for achieving a short total length since an enlargement magnification is provided by adopting a negative lens group as the fourth lens group. The three-group zoom lens is suitable for achieving a compact and low-cost structure since the number of lens groups is small.
However, the four-group afocal zoom lens has a fixed total length from the wide-angle end to the telephoto end, and it is difficult to shorten the total length. For this reason, the four-group afocal zoom lens tends to be large in size and heavy, and is not advantageous in portability. On the other hand, in the double telephoto type zoom lens, the total length at the wide-angle end can be smaller than that at the telephoto end, and portability can be improved. However, since the double telephoto type zoom lens requires four lens groups, the lens barrel structure is complicated, and cost cannot be sufficiently reduced. The three-group zoom lens (3) comprising positive, negative, and positive lens groups is suitable for achieving a low-cost structure, and can shorten the total length at the wide-angle end since the number of lens groups is as small as three. Also, this zoom lens is advantageous in portability. Such a zoom lens is proposed by, e.g., Japanese Patent Application Laid-Open No. 63-118116.
As a conventional focusing method of a zoom lens, a one-group extension method for moving the first lens group is known. This one-group extension method has a merit that moving amounts upon focusing to a given distance are equal to each other at the wide-angle end and the telephoto end, and is conventionally widely used as a focusing method of a zoom lens.
However, with the one-group extension focusing method, when an object at a near distance is to be focused, since the first lens group largely moves in the object direction, the vignetting in such a near distance phototaking state tends to be large, and in order to obtain a small vignetting even in the near distance phototaking state, the size of the first lens group must be increased. For this reason, this results in an increase in lens diameter and an increase in size of the entire zoom lens. Furthermore, when a zoom lens adopting this focusing method is attached to an auto-focus type camera and an auto-focus operation is to be performed, the first lens group as a large, heavy lens group must be moved, and it is difficult to achieve high-speed focusing.